1. Field the Invention
The invention relates to a variable force support device for supporting an in-line pipe inspection tool during longitudinal travel of the tool through a pipe being inspected.
2. Description of the Prior Art
It is known to perform in-line inspection of pipe by magnetic flux leakage technology. With these inspection practices, an in-line pipe inspection tool is propelled through the pipeline by the flowing product therein, which for example may be oil. As the tool passes through the pipeline, a strong magnetic field is induced into the pipe wall. Defects in the form of discontinuities will cause redistribution of the magnetic flux around the defect. This results in some of the lines of magnetic flux leaking out into the surrounding medium. The inspection tool embodies an electromagnet which is battery powered to power an electromagnet that induces the magnetic flux field into the pipe wall. Two sets of steel brushes are used in conjunction with the electromagnet to constitute the magnetic north and south poles of the magnetic flux field. It is necessary to maintain constant contact between the two sets of brushes and the internal surface of the pipe to ensure an uninterrupted magnetic flux field within the pipe wall. This is difficult to achieve if a single inspection device is used with pipe of different diameters. A plurality of transducers are used to detect magnetic flux leakage indicating a defect in the pipe wall. The transducers provide a signal indicating the presence of a defect, which signal may be processed by various electronic recording arrangements.
The two sets of steel brushes employed with the magnetizer act to support the inspection tool during its travel through the pipe. After extended use of the inspection tool, these steel brushes tend to wear and deform as a result of the combination of the weight of the inspection tool being supported, the weight of the fluid in the pipe above the inspection tool and contact with the interior surfaces of the pipe. This results in misposition of the inspection tool during travel through the pipe and discontinuous contact between the steel brushes and the interior pipe wall surface. This adversely affects the desired uninterrupted magnetic flux field in the pipe wall necessary for effective defect detection. This condition is exacerbated when the tool is used with pipes of different diameters. With these applications the brushes are required to extend sufficiently to support the tool with increased diameter pipe and then compress when inspecting smaller diameter pipe. During extension of the brushes, there must be sufficient force exerted by the brushes against the pipe wall to provide the required contact to ensure an uninterrupted magnetic flux field within the pipe wall.